Ceramic pressure sensors comprise a platform and a measuring membrane, wherein the measuring membrane is joined with the platform by means of an active hard solder, or braze. A suitable active hard solder, or braze, for joining ceramic parts, where the ceramic is corundum, is, for example, a Zr—Ni—Ti alloy, since its coefficient of thermal expansion is compatible with that of corundum.
European Patent EP 0 490 807 B1 discloses such an active hard solder, or braze. The patent EP 0 558 874 B1 discloses a method for manufacturing rings of such an active hard solder, or braze. For connecting measuring membrane and platform, rings of such active hard solder, or braze, are placed as space holder between the two parts and melted in a high vacuum soldering, or brazing, process, whereby a pressure tight and high strength, annular connection between the two ceramic parts arises. An alternative for applying the solder in the form of prefabricated rings is to provide the active hard solder, or braze, in a screen printing method. A screen printable paste of the active hard solder, or braze, and a method for its manufacture is disclosed in Offenlegungsschrift EP 0 988 919 A1.
From the desire for miniaturization of pressure sensors, there follows indirectly the need for a thinner joint, since—, for example, in the case of a ceramic pressure sensor with a capacitive transducer—the miniaturization leads to a reduction of the electrode areas of the capacitive transducer, which then needs to be compensated by a lessening of the spacing, when the capacitance should remain unchanged.
The rings can, however, only be manufactured with reproducible thickness at a minimum thickness of, for instance, 30 μm, so that thinner joints with such active solder rings cannot be manufactured with sufficiently small tolerances.
Equally, the use of a screen printable paste leads, due to grit size of the solder, to a minimum joint thickness of 30 μm, when a sufficient homogeneity of the joint and a low tolerance of the distance between the ceramic parts should be achieved.
The as yet unpublished application DE 10 2010 043119 discloses a method for manufacture of a pressure sensor, wherein the entire material of the joint is prepared by means of gas phase deposition. Although therewith thin joints can be prepared to the extent desired, it is nevertheless a comparatively expensive method, since in the case of the gas phase deposition naturally only a fraction of the evaporated material reaches the substrate.